1. Field of the Invention
The present invention relates to an insulated wire and to a coil and a motor that are formed using the insulated wire.
2. Description of the Related Art
Motors are used in drive units of compressors for refrigeration machines and compression machines, vehicles, and the like. A drive unit of compressors, vehicles, and the like includes a container having a high sealing property and incorporates a motor housed in the container. The container may include an air breather in order to compensate for variations in internal pressure due to temperature rise. Thus, the motor is hermetically or substantially hermetically sealed in the container of the drive unit.
The motor used in the drive unit includes a coil. The coil is formed by winding an enamelled wire (insulated wire). The insulated wire includes a conductor and an insulating film composed of an insulating paint on the periphery of the conductor. The insulating film is required to have an insulating property or the like and is composed of polyester, polyester imide, polyimide, or the like.
Such a drive unit requires strict moisture control in the container that houses the motor. Even a trace amount of moisture present in the container reduces the insulating property of the motor with the operation time of the motor, which results in poor insulation. Specifically, operation of the drive unit increases the temperature inside the container, and accordingly the internal pressure of the container increases. Therefore, if moisture is present in the container, the reactivity of the moisture is increased, and the moisture causes hydrolysis of the insulating film of the insulated wire that constitutes the coil of the motor and thereby degrades the insulating film. The insulating film gradually becomes degraded with the operation time of the drive unit, which reduces the insulating property of the motor. As a result, dielectric breakdown occurs. Even when the container includes an air breather in order to control the internal pressure of the container, moisture is likely to enter the container through the air breather. This promotes hydrolysis of the insulating film due to the temperature rise in the container, and thereby dielectric breakdown occurs.
In this respect, in order to reduce the amount of moisture entering a container including an air breather, a method in which a moisture absorbent is installed in the air breather has been proposed (e.g., Japanese Unexamined Patent Application Publication No. 8-29257).